1. Field of the Invention
The present invention generally relates to a process for making blends or composites of two or more polymers wherein a continuous fibrillar morphology is formed in-situ within a matrix. The invention has particular application in producing in situ fibrillar liquid crystal polymer reinforced thermoplastic materials.
2. Description of the Prior Art
Reinforced polymer composites are well known and widely used. By combining appropriate fiber or particulate reinforcing materials with a polymer matrix, improved strength and stiffness qualities are achieved. Examples of reinforcing material fillers include glass fibers, carbon fibers, aramid fibers, mica, calcium carbonate and talc. Examples of base matrix polymers which have been compounded with reinforcing material fillers include a wide range of thermoplastics such as polyethylene, polypropylene, polystyrene, polyamides and polyesters such as poly(ethylene terephthalate). In addition, it is known to combine reinforcing material fillers with high performance polymers, which are often referred to as engineering thermoplastics, such as poly(ether ether ketone) (PEEK), poly(phenylene sulphide) (PPS), polycarbonate, and the like.
Recently, there has been much research directed toward developing "self-reinforced" polymer composites which would comprise continuous and unidirectional fibers of one material which are formed in-situ in a thermoplastic matrix material. A difficulty which must be overcome in the production of "self-reinforced" polymer composites is that, with some exceptions, most polymer blends are immiscible and consequently form two phases upon mixing. In general, one component forms the continuous phase (matrix component) while the other phase is dispersed (dispersed component) in the matrix. The morphology of the blend, which concerns the size, shape and degree of the dispersion, depends on a number of different factors such as viscosity ratio, composition ratio, interfacial tension, temperature, type of flow field, elasticity ratio, residence time, intensity of mixing, and the like. A combination of some or all of these factors are often referred to as processing conditions. Different morphologies with the same polymer-polymer system may be obtained under different processing conditions. In the case of the in situ composites made with liquid crystal polymer (LCP) materials, the desired morphology would be to have the LCP phase form long, continuous fibrils in the thermoplastic matrix.
Formation of long, continuous LCP fibrils would provide effective reinforcement of a thermoplastic matrix and would result in the composite having enhanced strength and stiffness properties. However, it is well known that the viscosity of the LCP phase should be lower than that of the matrix phase during processing in order to produce the LCP fibrils. Furthermore, once the LCP fibrils are formed, they will be stable, meaning they will not break up into droplets, for longer periods of time typically only if the magnitude of the matrix viscosity itself is high. These conditions are difficult, if not impossible, to achieve when blending certain thermoplastic/LCP polymer pairs in the same apparatus (e.g., single screw extruder or twin screw extruder).
Despite these complications, fiber formation in thermoplastic matrixes has been achieved for particular thermoplastic/LCP polymer pairs. Blizard and Baird, in Polym. Eng. Sci., Vol. 27, No. 9, (1987), studied blends of PET/60 PHB (a liquid crystalline copolyester of 60 mol % p-hydroxybenzoic acid and 40 mol % poly(ethylene terephthalate) with nylon 6,6 or polycarbonate (PC). The Blizard work indicated that LCP microfibrils can be generated at certain compositions, particularly under the influence of extensional flows. Weiss et al., in Polym. Eng. Sci., 27, 684 (1987), extruded fibers (strands or filaments) of blends of a low molecular weight LCP with polystyrene, with the LCP composition being less than 10 weight percent in the blend, and reported some enhancement (ca. 50%) in the tensile properties of the fibers due to the reinforcing effect of the LCP fibrils.
Further discussions on the morphology and mechanical properties of LCP blends may be found in a recent review by Dutta et al., in Polym. Eng. Sci., 30(17), 1005 (1990). In a majority of the studies, the mixing or blending was done either in a single screw extruder or a single screw extruder in series with a static mixer or a twin screw extruder or in an injection molding unit. The function of these mixing apparatuses is to provide a good dispersion of the minor component in the matrix. However, the dispersion of the LCP in the matrix created by the single screw extruder or twin screw extruder is in the form of LCP droplets. Further extensional deformation, such as that provided in the converging section of capillary dies and/or that provided by drawing at the die exit, is required to convert these LCP droplets into elongated structures. These elongated structures may then in turn form more continuous, fibrillar structures provided the concentration of the dispersed phase is high enough to allow the individual elongated structures to coalesce and thus attain more continuity. Typically, the LCP phase should constitute at least 10-15 weight percent of the blend in order for the droplets to be able to elongate and join for fiber formation at the die. In addition, the morphology of the blends formed by extrusion out of a single or twin screw extruder often have a skin-core type of structure with fibrous LCP present in the skin region and LCP droplets in the core region. This morphology is not very desirable since the LCP droplets in the core do not contribute to the mechanical property enhancement of the matrix material and, thus, represent less than optimal reinforcement.
Isayev and Modic in European Patent Application 0,217,563 (1986) and Isayev and Swaminathan in European Patent Application 0,291,323 (1988) disclose the in-situ formation of LCP or aromatic polyester fibers in thermoplastic matrix materials, respectively. In both Isayev et al. patent applications, the matrix and fiber forming materials are blended together in a single screw or twin screw extruder in series with a static mixer. Because the materials are processed simultaneously, the processing temperatures of the fiber-forming phase and the matrix forming phase must overlap, otherwise, the matrix forming phase will be severely degraded during processing.
U.S. Pat. No. 4,547,541 to Golba and European Patent Application 0,340,655 (1988) to Federici et al. are both directed to blending processes where two or more components of the blend are melted separately and then combined in an extruder. Melting polymers separately solves the situation where the polymer materials have incompatible processing temperatures, especially if, as taught in Federici, an LCP, which is capable of supercooling such that it is molten at a temperature below its melting point after melting, is to be combined with a lower melting point matrix polymer. In Golba, the melt from a single screw extruder is fed directly into a twin screw extruder at a point down stream from the feed hopper of the twin screw extruder. The two molten compositions are then blended in the remaining portion of the twin screw extruder. In Federici et al., the melt feed from one extruder is fed directly into another extruder containing the other thermoplastic melt. A problem with both the Golba and Federici et al. methods is that they rely on an extruder to mix the melted materials. As discussed above in conjunction with Dutta et al. review in PoIym. Eng. Sci., 30(17), 1005 (1990), an extruder primarily provides a dispersion of one material with another (although some distribution is achieved), and if an LCP is being blended with a thermoplastic material, the LCP will be present as droplets in the blend exiting from the extruder. While the droplets may be elongated to form fibers using a die or perhaps even a down stream static mixer, the fibers typically have a skin-core type of structure where the skin is fibrous but the core is still in the form of an LCP droplet. Such fibers do not optimally reinforce the matrix materials. In addition, there are practical limits to the amount of LCP required to form these fibers. Because an extruder operates by moving the stream forward while simultaneously causing a portion to move horizontally, the LCP droplets are separated from each other. Hence, upon elongation there must be a sufficient amount of LCP present to allow the elongated droplets created by the die to touch and form a fiber. Typically, a minimum of 10-15 wt % of the blend should be LCP before fibers will form.